The King has made his first statement since being diagnosed with cancer.
In a message, written from Sandringham in Norfolk, King Charles said: "I would like to express my most heartfelt thanks for the many messages of support and good wishes I have received in recent days.
"As all those who have been affected by cancer will know, such kind thoughts are the greatest comfort and encouragement."
As well as thanking the public for their well wishes, the 75-year-old monarch also reflected on his decision to publicly reveal his illness, saying: "It is equally heartening to hear how sharing my own diagnosis has helped promote public understanding and shine a light on the work of all those organisations which support cancer patients and their families across the UK and wider world."
Stressing his gratitude to the medical professionals and cancer charities, he added: "My lifelong admiration for their tireless care and dedication is all the greater as a result of my own personal experience."
On Monday, Buckingham Palace announced the King had been diagnosed with a form of cancer while undergoing a procedure for an enlarged prostate.
In line with the initial announcement, the King in his new statement has not revealed any further details about what kind of cancer it is, the type of treatment he is having or what stage the cancer is at.
He started his treatment in London on Monday, and on Tuesday flew to Sandringham with the Queen.
It's unclear when he may return to London, or if he'll need to come back to the capital for his treatment.
The palace and the monarch had hoped that by being open about his cancer they could stop any potential rumours or speculation and allow the King to have some level of privacy while undergoing treatment.
In whatever ways they can they'll be keen to offer reassurance that he remains capable of carrying out his constitutional duties.
This statement will be part of that strategy to show he remains positive. But they have made it clear they won't be drawn into giving a running commentary about his condition.
On Thursday, at an event at Salisbury Cathedral, the Queen spoke about her husband's diagnosis for the first time, telling Ben Abbott, a critical care paramedic: "He is doing extremely well under the circumstances. He is very touched by all of the letters and messages the public have been sending from everywhere. That's very cheering."
During a speech at an Air Ambulance event on Wednesday night, Prince William also took time to thank everyone for their support both for his father and the Princess of Wales, who is recovering from abdominal surgery.
Other members of the Royal Family are now having to carry out the public engagements, with the monarch told not to do public-facing duties by doctors.
It makes a written statement from the King even more significant, as it is his main way of communicating for now.